A number of difficult technical problems are common between ripple free wifi and power-line communication, notably those of spread spectrum radio signals operating in a crowded environment. Radio interference, for example, has long been a concern of amateur radio groups.
Power-line communications systems operate by adding a modulated carrier signal to the wiring system. Different types of power-line communications use different frequency bands. The main issue determining the frequencies of power-line communication is laws to limit interference with radio services. Many nations regulate unshielded wired emissions as if they were radio transmitters. These jurisdictions usually require unlicensed uses to be below 500 KHz or in unlicensed radio bands.
Data rates and distance limits vary widely over many power-line communication standards. Utility companies use special coupling capacitors to connect radio transmitters to the power-frequency AC conductors. These signals may be impressed on one conductor, on two conductors or on all three conductors of a high-voltage AC transmission line. On some powerlines in the former Soviet Union, PLC-signals are not fed into the high voltage line, but in the ground conductors, which are mounted on insulators at the pylons. While utility companies use microwave and now, increasingly, fiber optic cables for their primary system communication needs, the power-line carrier apparatus may still be useful as a backup channel or for very simple low-cost installations that do not warrant installing fiber optic lines.
110 kV, 220 kV, 400 kV. The modulation generally used in these system is amplitude modulation. The carrier frequency range is used for audio signals, protection and a pilot frequency. The pilot frequency is a signal in the audio range that is transmitted continuously for failure detection. The voice signal is compressed and filtered into the 300 Hz to 4000 Hz range, and this audio frequency is mixed with the carrier frequency. The carrier frequency is again filtered, amplified and transmitted. This range is set according to the distance between substations.
Wave traps are used in switchyard of most power stations to prevent carrier from entering the station equipment. A coupling capacitor is used to connect the transmitters and receivers to the high voltage line. This provides low impedance path for carrier energy to HV line but blocks the power frequency circuit by being a high impedance path. The coupling capacitor may be part of a capacitor voltage transformer used for voltage measurement. Power-line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control.
PLC is one of the technologies used for automatic meter reading. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history—not so much because there is an interest in automating a manual process, but because there is an interest in obtaining fresh data from all metered points in order to better control and operate the system. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change. Outbound messages injected at a utility substation will propagate to all points downstream. Power-line communications technology can use the electrical power wiring within a home for home automation: for example, remote control of lighting and appliances without installation of additional control wiring. The carrier is modulated by digital signals.
Each receiver in the system has an address and can be individually commanded by the signals transmitted over the household wiring and decoded at the receiver. The physical layer method is a very different scheme than the X10. Narrowband power-line communications began soon after electrical power supply became widespread. Consumer products such as baby alarms have been available at least since 1940. For many years the search continued for a cheap bi-directional technology suitable for applications such as remote meter reading.